Field of the Invention
The present invention relates to an epitaxial substrate having a group 13 nitride layer epitaxially formed on a base substrate, and more particularly to an epitaxial substrate for an HEMT device.
Description of the Background Art
A group 13 nitride epitaxial substrate for an HEMT device in which an HEMT structure is formed of a group 13 nitride epitaxial film on a SiC substrate is already known in the art as a high electron mobility transistor (HEMT) device. For example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-520880 discloses an epitaxial film having an HEMT structure in which an AIN buffer layer and an insulating GaN layer are formed in this order on a semi-insulating SiC substrate.
In order that an HEMT device may have, for example, a high performance such as high output, low loss and high-frequency operation as well as a high reliability, the HEMT device must be provided with characteristics such as high mutual conductance, high maximum drain current, low on-resistance, low leakage current, low current collapse, high cut-off frequency and high voltage resistance. Further, in order that these characteristics may be realized, it is demanded that a group 13 nitride epitaxial substrate for the HEMT device has characteristics such as high electron mobility, high sheet carrier density, low surface roughness and high voltage resistance.
With respect to this point, regarding an epitaxial wafer for a high-frequency device such as an HEMT, in the case in which a GaN-based semiconductor layer is grown on an SiC base substrate, it is already known in the art that a sheet resistance variation within the same growth batch or under the same growth conditions can be reduced by setting an off angle of the SiC base substrate to be 0° or more and 0.2° or less. For example, reference is made to Japanese Patent Application Laid-Open No. 2013-187368.
With reference to the disclosed contents of Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-520880 and Japanese Patent Application Laid-Open No. 2013-187368, the inventors of the present invention fabricated a group 13 nitride epitaxial substrate for an HEMT device by laminating an AlN nucleation layer, a GaN electron transit layer, an AlN spacer layer and an InAlN barrier layer sequentially on a semi-insulating SiC substrate in which the main surface was a (0001) plane and an off angle was 0.1°, and evaluated the characteristics thereof. However, the obtained characteristics of the extent that the electron mobility μ was about 500 cm2V−1s−1 were insufficient. Also, numerous fine pits were found to be generated on a surface of the barrier layer of the obtained epitaxial substrate, suggesting that the presence of such pits had a relationship of some sort to the lowering of the electron mobility.
By considering these results, the inventors of the present invention have made eager studies and have obtained a knowledge that, in order to fabricate a group 13 nitride epitaxial substrate for an HEMT device having superior characteristics, particularly a superior electron mobility, and further to fabricate an HEMT device having superior characteristics, for example, a superior maximum drain current, it is effective to provide an intermediate layer composed of AlGaN between the AlN nucleation layer and the GaN electron transit layer and to adjust the setting of the off angle of the semi-insulating SiC substrate.